MacCHESS, a synchrotron radiation Research Resource for macromolecular crystallography, has the overall goal of advancing the frontiers of structural biology through innovative technical research and development and user support. MacCHESS leverages the National Science Foundation investments in the Cornell High Energy Synchrotron Source (CHESS), which maintains the synchrotron radiation laboratory, and the Laboratory for Elementary Particle Physics (LEPP), which operates the storage ring. MacCHESS technical developments are driven by both Core and Collaborative research projects involving a broad range of macromolecules. In addition to performing technical R&D and Core research, MacCHESS provides specialized instrumentation for macromolecular crystallography and a staff for user training and support. MacCHESS has established itself as one of the most productive facilities in the world for macromolecular crystallography, with almost 437 papers published during the past five years as a result of MacCHESS related technical research and development, Core and Collaborative research and Service. More than 72 of these papers were published in the high visibility journals, Science, the Nature journals, and Cell. Three CHESS stations, A-1, F-1 and F-2, are used for macromolecular crystallography experiments. During the next five years, MacCHESS technical R&D will focus on (1) microcrystallography, (2) membrane protein crystallography, (3) large unit cell structures, (4) ultrahigh resolution diffraction, (5) high throughput crystallography, (6) crystallographic software and phasing techniques and (7) crystallographic beamline hardware development. Each of these technical R&D efforts is driven by the challenging problems presented by collaborating investigators who study the atomic structure of molecules and complexes of great current interest to medicine and biology, including membrane proteins, regulatory protein complexes, nucleic acid/protein complexes, protein drug interactions and viruses. Technical developments resulting from MacCHESS research are freely available to the scientific community and disseminated through meetings, workshops, publications, web distributions and collaborations with other synchrotron sources.